CVE-2026-68166 userfaultfd: prevent registration of special VMAs
A local attacker can exploit userfaultfd on shadow stack VMAs to inject return addresses, leading to privilege escalation or code execution.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-68166?
CVE-2026-68166 is a vulnerability in Linux, published on 10 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
userfaultfd: prevent registration of special VMAs
Vova Tokarev says:
userfaultfd allows registration on shadow stack VMAs. With userfaultfd access, you can register on the shadow stack, discard a page ... and inject a page with chosen return addresses via UFFDIO_COPY.
Update vma_can_userfault() to reject VM_SHADOW_STACK.
While on it, also reject VM_SPECIAL so that if a driver would implement vm_uffd_ops, it wouldn't be possible to register special VMAs with userfaultfd.
Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb, exclude hugetlb VMAs from the check for VM_SPECIAL.
Which versions are affected by CVE-2026-68166?
How do I fix CVE-2026-68166?
Apply the vendor's patch.
Is CVE-2026-68166 being exploited?
There is no known exploitation of CVE-2026-68166 in the wild and no public exploit code.
Sources
Also known as AZL-94734, BELL-CVE-2026-68166, DEBIAN-CVE-2026-68166, GHSA-hv63-47wx-6p3x, UBUNTU-CVE-2026-68166.